1. Field of the Invention
The present invention relates to radiation detectors and particularly to a solarblind radiation detector that responds only to ultraviolet light at wavelengths below about 285 nanometers in the presence of solar illumination.
2. Description of the Related Art
The detection of ultraviolet (UV) light during daylight conditions is an important problem for both commercial and military applications. It is difficult to design a very sensitive detector that can be used in broad daylight to detect very low levels of UV radiation. The spectral distribution of radiation from the sun is similar to that of a 6,000 degree black-body radiator. The solar spectral distribution drops off very sharply below 290 nm due to atmospheric absorption by ozone. As a result, the earth's surface is essentially dark below 290 nm. A solar-blind detector can be defined as a device or apparatus that only responds to light wavelengths below about 285 nm.
Applications for solar-blind detectors include monitoring lightning events during thunderstorms, detecting ultraviolet laser sources such as excimer lasers or frequency quadrupled Nd:YAG lasers used as LIDAR sources, and ultraviolet telescope detectors for space platforms.
Many prior art approaches have been proposed to achieve solar blind detector performance.
One approach, described in U.S. Pat. No. 4,731,881, uses a series of chemical and color glass filters to accomplish UV transmission below 285 nm and a sharp cut off, blocking wavelengths longer than 285 nm. The chemical filters consist of an expensive, single crystal nickel sulfate hexahydrate crystal that has very poor thermal and moisture stability, and an organic dye, Cation X, contained in a polyvinylalcohol film to provide UV bandpass characteristics. This approach uses a relatively expensive UV sensitive photomultiplier tube for detection.
Another approach (described in U.S. Pat. No. 4,731,881) uses a ruby crystal with interference filters coated on the two faces. The input face has a bandpass interference filter that transmits a narrow UV band at approximately 254 nm and rejects all other wavelengths. The output face of the ruby crystal is coated with an interference filter that transmits the ruby fluorescence wavelengths and blocks all other wavelengths. The performance of this device is limited by the band pass and broad band blocking capability of interference filters. A dielectric coating is limited to a rejection of about 10.sup.5 outside of the bandpass region. An out-of-pass-band rejection of approximately 10.sup.18 is necessary for true solar blind detection.
Other approaches (described in U.S. Pat. Nos. 4,241,258 and 5,331,168) use UV sensitive phosphor powders as downconverters. Phosphor powders are highly scattering and can result in reduced light collection efficiency.
Such prior art approaches suffer from either excessively low transmission in the UV signal wavelength region or inadequate rejection of visible light.